1. Field of the Invention
A method used in a wireless communications system and related communication device are disclosed, and more particularly to, a method of handling downlink signaling in a wireless communications system and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred as user equipments (UEs).
Generally, the UE performs measurements for positioning, channel quality indication and mobility. Measurement is done by monitoring various types of reference signals. Positioning Support in LTE has been agreed as a working item in 3GPP RAN Plenary meeting #42 and is focusing on the downlink terrestrial positioning method where one objective is that positioning for LTE access needs to be as good as or better than that currently possible for other access types. In addition, all new features and capabilities shall be backward compatible with networks and UEs that support Rel-8 LTE and Evolved packet System (EPS). An Observed Time Difference of Arrival (OTDA) is specified as a method for positioning. The OTDOA measurement is based on the arrival time difference from different cells observed/measured at the UE. The accuracy of position estimates depend on the precision of timing measurements.
OTDOA time difference measurement is based on a reference signal (RS) and there should be an ability to provide assistance data to the UE containing at least physical cell identities (PCIs) of candidate cells for measurement, relative transmission timing of PCI candidates to a serving cell, and the candidate cell set may be different from the neighbor cells considered for handover. On the other hand, positioning subframes are based on normal subframe with no data or MBSFN subframe with no data and/or RS in the data region while semi-statically configuring positioning subframe for cells/group of cells where UE with positioning subframes can assume aligned, partially or non aligned positioning subframes from all cells as indicated in the assistance information.
A long term evolution-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system, considering relaying for cost-effective throughput enhancement and coverage extension. For example, a relay can be deployed at the cell edge where the eNB is unable to provide required radio quality/throughput for the UEs or at certain location where radio signals of the eNB cannot cover. The LTE-A system can support a wider bandwidth up to 100 MHz to satisfy requirement for peak data rate. Coordinated multi-point (CoMP) transmission/reception is considered for the LTE-A as a tool to improve the coverage of high data rates, the cell-edge throughput and/or to increase system throughput. The main idea of the CoMP transmission/reception is as follows: when a UE is in the cell-edge region, it may be able to receive signals from multiple cell sites and the transmission of the UE may be received at multiple cell sites regardless of the system load. Given that, if the signaling transmitted from the multiple cell sites is coordinated, the DL performance can be increased significantly. This coordination can be simple as in the techniques that focus on interference avoidance or more complex as in the case where the same data is transmitted from multiple cell sites. For the UL, since the signal can be received by multiple cell sites, if the scheduling is coordinated from the different cell sites, the system can take advantage of this multiple reception to significantly improve the link performance.
Dedicated reference signal (DRS) is used at a specific antenna port and a transmission mode for demodulation. For a UE in a radio resource control connection mode and/or CoMP operation, it is time-inefficient to read positioning configuration and wait for positioning RS opportunity. For channel quality measurement, channel state information reference signal (CSI-RS) is expected to improve measurement accuracy in LTE-A. However, the CSI-RS location is not defined for LTE-A. Without careful design and scheduling, radio resource could be wasted and interference is caused.
A Hybrid Automatic Repeat-request (HARQ) operation, done in combination between a media access control (MAC) layer and a physical (PHY) layer, retransmits transport blocks (TBs) for error recovery. The PHY layer performs the retention and re-combination (incremental redundancy) and the MAC layer performs the management and processing negative acknowledgement/acknowledgement (NACK/ACK) signaling. Two HARQ feedbacks (NACK/ACK) indicate whether the eNB has correctly received a transmission on physical uplink share channel (PUSCH). When the NACK is received, or when a certain time elapses without receiving any feedback, the transmitter retransmission the transport block. A measurement gap disturbs the HARQ feedback reception in two different ways: either the transmission of the previous feedback could not take place because of the measurement gap or the previous transmission itself could not take place because of the measurement gap. Those two cases are respectively handled by the two following rules:
1) if a previous transmission takes place but its HARQ feedback cannot be received because of a measurement gap: consider the HARQ feedback as an ACK; and
2) if the measurement gap occulted the previous transmission itself: refer to the last received HARQ feedback.
In the LTE system, a measurement gap is 6 ms at DL and 7 ms at UL with occurrence periodicity of 40 ms or 80 ms. If 3 ms is assumed for UE processing time, 9 ms or 10 ms in total should be considered. For half duplex with 4 ms fixed pattern for uplink/downlink, it is the most likely the measurement gap collide with one or more (re)transmission. However, the current specification only deals with one (re)transmission and/or its corresponding HARQ feedback. On the other hand, it has been unknown what happens to the initial transmission itself which collides with measurement gap since no previous HARQ feedback is referred for (non-adaptive) retransmission opportunity.